The invention relates to a filling pipe for a fuel tank of a motor vehicle having a fuel vapor absorbing system, wherein the filling pipe has a reduced cross section over part of its length. By fuel vapor absorbing system is meant a system which absorbs fuel vapors which are displaced out of the fuel tank during refueling and feeds them to a regenerable filter. Such a system is known by experts as an ORVR system (Onboard Refueling Vapor Recovery).
In order to carry out this recovery function properly, it must be ensured that during refueling the fuel vapors do not flow out through the filling pipe, but rather flow in their entirety through the filter. For this purpose, the fuel which flows through the filling pipe during the refueling procedure has to build up a sealing liquid column which forms a liquid seal and thereby prevents fuel vapors from escaping through the filling pipe.
According to the prior art, the liquid seal is achieved by a circular symmetrical constriction of the filling pipe over part of its length. A venturi nozzle-like effect is therefore produced. However, this liquid seal fails if a turbulent flow forms in the filling pipe, whether due to a fuel nozzle inserted crookedly or due to the Coriolis force similar to drainage from a bathtub. In case of a helical flow being produced in this manner, the liquid is pressed by the centrifugal force against the pipe wall, and in the vicinity of the longitudinal axis a space is formed which is not filled with liquid and through which fuel vapors escape to the outside. This known design also has the disadvantage that the reduction in diameter of a pipe made of metal is costly in terms of manufacturing because it is normally done by expanding the adjacent parts.